How to render separately multiple objects on the same scene, without using multiple render layers?
As far as I understand, you need to, under one scene, render more than 20 object separately, here's one solution :


under the object tab, in the properties panel, assign each of your object a single Pass index, "1" for the first object, "2" for the second, etc...





Once this done, go to Node Editor, select the compositing node tree and choose use node 





Under layers tab, activate de the object index pass





connect the IndexOb pass to an Id Mask node. Make sure the pass index in the Id Mask node correspond each time to your pass index's object. Connect the Id Mask node to the fac of a Mix node, &amp; connect the image from the render layer node to the second input image of the mix node, as shown in the image below. Repeat this process for every object you want to render separately (and don't forget to change the ID mas node number to your corresponding object)





You can repeat this process as long as you want, with all you objects, and then assign a different output location to each ones, with the file output node :






EDIT : Actually, this solution isn't perfect, because if other objects are in front of the rendered object, the overlapping part will mask the second...
Another solution, a lot simpler, is simply to uncheck the render button in the Outliner panel, but you'll need to check and uncheck for the corresponding object every time you want to render a single object, and change the folder each time too.